The retreading, renewing or rebuilding of large, off-the-road, heavy duty tires has departed in recent years from the classic prior procedure referred to as mold curing. Mold curing, as known to persons in the tire industry, included steps of applying uncured rubber to a tire carcass and then inserting the carcass into a configured mold. The tire carcass and uncured rubber were subjected to elevated pressures and temperatures to shape the uncured rubber into a desired tread configuration, cure the previously uncured rubber, and adhere the new tread material to the tire carcass.
Techniques which have replaced mold curing for large, off-the-road, heavy duty tires are exemplified by Ragan U.S. Pat. No. 27,006 and Ragan U.S. Pat. No. 3,472,714. The first mentioned patent, Ragen U.S. Pat. No. 27,006, describes a method in which discrete lugs are formed and are then applied to the tread face of a tire undergoing renewing or rebuilding. The second, Ragan U.S. Pat. No. 3,472,714, describes a method in which a body of rubber is applied to the tread face of a tire undergoing renewing or rebuilding and portions of the rubber are then removed with a heated knife to form lugs of the remainder.
Of the two processes briefly described, the first to achieve commercial significance, and to substantially replace classic mold curing, was the discrete lug process described for example in Ragan U.S. Pat. No. 27,006. Such processes were found to be significant improvements over prior practices, particularly in the area of reduced carcass damage during the renewing or rebuilding process and consequent extended carcass life.
Discrete lug processes, such as those of Ragan U.S. Pat. No. 27,006, suffer from a relatively high labor requirement. That is, the number of man-hours required to be spent in the renewing or rebuilding of a particular size tire carcass is greater than the man-hours required by classic mold curing. For some time, the higher labor cost involved was acceptable due to the substantially better result achieved. More recently, the cost of the higher labor requirement has become more burdensome, and development has turned toward processes such as the cut tread process described in Ragan U.S. Pat. No. 3,472,714.
As will be noted from the disclosure of that prior Ragan patent and other patents known to persons skilled in the applicable art, the apparatus used for cutting tread configurations was relatively simple and almost crude during the early stages of development of such processes. More particularly, the heated knives used were operated manually and were either guided manually or guided by relatively simple and unsophisticated mechanical mounting arrangements. While such approaches to cut or groove tread processes were able to clearly demonstrate the commercial viability of such a process, and show the promise of reducing the labor requirement for renewing or rebuilding tires of the type described, the manual or semi-manual apparatus available and employed imposed certain restrictions on the processes. More recently, development has turned toward grooving or cutting apparatus and methods having greater flexibility and adaptability, as shown for example in Appleby et al U.S. Pat. No. 4,081,017. While such attempted mechanization or automation of such cutting processes has improved upon the relatively unsophisticated and crude apparatus used in the earliest stages of development of cut tread processes, proper operation of such an apparatus still requires a substantial level of skill from an operator. Further, such an apparatus is mechanically complex, and does not assure that particular tread configurations are repeatedly and consistently achieved. Finally, such arrangements are relatively inflexible and are not readily adapted to variations in tire carcass dimensions or in patterns of tread design. These difficulties and deficiencies result in less than optimal realization of the benefits available, such as by resulting in excessively thick or excessively thin undertread layers of newly applied rubber in a renewed or rebuilt tire. Where an undertread portion is too thick, excessive rubber is used and overheating failure may occur. Where too thin, failure occurs due to separation, tearing or puncturing. Sometimes, inaccurate control destroys a tire carcass by cutting into the plies of the main carcass body.